In the formation of integrated circuits, semiconductor devices are formed on semiconductor substrates, and are then connected through metallization layers. The metallization layers are connected to the semiconductor devices through contact plugs. Also, external pads are connected to the semiconductor devices through the contact plugs.
Typically, the formation process of contact plugs includes forming an Inter-Layer Dielectric (ILD) over the semiconductor devices, forming contact openings in the ILD, and filling a metallic material in the contact openings. With the increasing down-scaling of integrated circuits, however, the above-discussed processes experience shortcomings. While the horizontal dimensions (for example, the poly-to-poly pitch between neighboring polysilicon lines) are continuously shrinking, the diameters of contact plug and the contact area between contact plugs to salicide are reduced. The thickness of the ILD is not reduced accordingly to the same scale as the reduction of the widths of the contact plugs. Accordingly, the aspect ratios of the contact plugs increase, causing the contact formation process to be increasingly more and difficult.
The down-scaling of integrated circuits results in several problems. First, it is increasingly more difficult to fill the contact openings without causing seam holes (voids) therein. In addition, when the lateral sizes of the contact plugs reduce, the sizes of seam holes do not reduce proportionally. This not only causes the effective area of the contact plugs for conducting currents to reduce non-proportionally, but also results in the subsequently formed contact etch stop layer and metal lines to fall into the seam holes, and hence results in reliability problems. As a result, the process window for forming the contact openings becomes narrower and narrower, and the formation of contact plugs has become the bottleneck for the down-scaling of integrated circuits.